1. Field of the Invention
The present invention generally relates to an in-phase (I) and quadrature (Q) receiver, and more specifically to an apparatus and method for compensating for IQ mismatch errors between the I and Q channels in an IQ receiver.
2. Background Art
Wireless communications systems often transmit data using an in-phase (I) and quadrature (Q) format. The I and Q channels in an IQ signal are phased-shifted relative to each other by 90 degrees, which is known as a quadrature relationship. The IQ format is popular for data transmissions because an IQ signal is capable of carrying two data streams in the frequency bandwidth that is normally required by a single data stream. In other words, the IQ format allows twice the data to be sent over a given frequency bandwidth. At the radio receiver, the I and Q channels are typically down-converted and demodulated to recover the I and Q channel data.
The IQ radio receiver typically includes separate receiver paths for the I channel and the Q channel. For instance, the I channel receiver path can include a first set of mixers, amplifiers, filters, etc. to down-convert and process the I channel data. Likewise, the Q channel receiver path can include a second set of mixers, amplifiers, filters, etc. to down-convert and process the Q channel data.
I/Q mismatch error in the radio receiver can impair the ability to successfully receive and process high speed data carried by the wireless signal. I/Q mismatch errors occur when the I channel gain is different from that of the Q
channel, or when the phase relationship between the two channels is not exactly 90 degrees. I/Q mismatch error is caused by gain and/or phase mismatches of the high frequency components in the I and Q channels of the I/Q receiver. For example, the receiver components in the I channel can have slightly different amplitude and/or phase characteristics than the receiver components in the Q channel, introducing mismatch errors in the I and Q baseband signals. Although the differences are usually small, these gain and phase mismatches reduce the effective signal-to-noise ratio of the IQ receiver, and increase the number of bit errors for a given data rate.
One conventional solution to reduce the effect of I/Q mismatch errors is to utilize highly precise components in the I and Q channels that have matching amplitude and phase characteristics. These high quality components may be too expensive for low cost applications, such as wireless local area network applications, or other low cost consumer applications.
Another conventional solution is to purchase lower precision components, but to test and characterizes the components so that components with matching amplitude and phase characteristics can be put in the same IQ receiver. Although effective, this solution is time consuming, and labor intensive because a technician must test and characterize each and every component. Therefore, this solution can be as costly as buying high precision components.
What is needed is an I/Q receiver that includes a means for detecting and correcting for I/Q mismatch error in real time, without resorting to high precision components or component characterization.